Key Strategies on Cu2O Photocathodes toward Practical Photoelectrochemical Water Splitting

Cuprous oxide (Cu2O) has been intensively in the limelight as a promising photocathode material for photoelectrochemical (PEC) water splitting. The state-of-the-art Cu2O photocathode consists of a back contact layer for transporting the holes, an overlayer for accelerating charge separation, a protection layer for prohibiting the photocorrosion, and a hydrogen evolution reaction (HER) catalyst for reducing the overpotential of HER, as well as a Cu2O layer for absorbing sunlight. In this review, the fundamentals and recent research progress on these components of efficient and durable Cu2O photocathodes are analyzed in detail. Furthermore, key strategies on the development of Cu2O photocathodes for the practical PEC water-splitting system are suggested. It provides the specific guidelines on the future research direction for the practical application of a PEC water-splitting system based on Cu2O photocathodes.


Introduction 1.Basic Principles of Photoelectrochemical Water Splitting
Photoelectrochemical (PEC) water splitting has been regarded as an ideal technology for generating CO 2 -free hydrogen because it does not emit CO 2 as a by-product during PEC operation, contrary to traditional hydrogen-production technologies based on fossil fuel.Figure 1 illustrates an operational principle of the PEC water-splitting system with two semiconductor electrodes.The overall PEC water-splitting reaction includes three steps [1][2][3][4]: (1) Electron-hole pairs are generated when sunlight is absorbed by the semiconductor in the water.(2) After separating the generated electron-hole pairs by the internal electric field in the depletion layer or the external bias, electrons in the conduction band (CB) of the semiconductor and holes in the valence band (VB) of the semiconductor drift to the semiconductor/water interface.(3) Water splits into hydrogen and oxygen by electrons and holes, respectively, at the semiconductor/water interfaces via the following reactions.Semiconductors are a key component in the PEC water-splitting system because it absorbs sunlight, and moreover water-splitting reactions occurs at its interface.There are two criteria of a semiconductor for overall PEC water splitting according to the operational principle of PEC water splitting, as illustrated in Figure 1 [1,[5][6][7][8].First, its CB energy level (E CB ) should be lower than the HER potential (0 versus reversible hydrogen electrode, RHE), while its VB energy level (E VB ) should be higher than the OER potential (1.23 V versus RHE).
E CB < 0 V versus RHE, E VB > 1.23 V versus RHE (4) Second, its band gap (E g ) should theoretically be larger than 1.23 eV because the difference between the HER potential and OER potential is 1.23 eV.Practically, an Eg of 1.5~1.8eV is necessary due to the HER/OER overpotentials (η HER and η OER ) [9,10].E g > 1.23 eV + η HER + η OER (5) level (ECB) should be lower than the HER potential (0 versus reversible hydrogen electrode, RHE), while its VB energy level (EVB) should be higher than the OER potential (1.23 V versus RHE).
ECB < 0 V versus RHE, EVB > 1.23 V versus RHE (4) Second, its band gap (Eg) should theoretically be larger than 1.23 eV because the difference between the HER potential and OER potential is 1.23 eV.Practically, an Eg of 1.5~1.8eV is necessary due to the HER/OER overpotentials (ηHER and ηOER) [9,10].
Among various semiconductors, few wide-band-gap semiconductors, such as TiO2 [11], ZnO [12], SrTiO3 [13], and Nb2O5 [14], meet these criteria, as shown in Figure 2a.Photoelectrodes with these materials can complete the entire PEC water-splitting reactions using only sunlight without external bias.However, they can absorb only ultraviolet light below 400 nm due to their band gaps above 3.0 eV, thereby utilizing the limited solar energy.Hence, PEC photoelectrodes based on these wide-band-gap semiconductors show a low solar-to-hydrogen (STH) efficiency below 3% (Figure 2b) [15].Therefore, semiconductors with a short band gap are necessary to utilize the sufficient solar energy for efficient PEC water splitting.Among various semiconductors, few wide-band-gap semiconductors, such as TiO 2 [11], ZnO [12], SrTiO 3 [13], and Nb 2 O 5 [14], meet these criteria, as shown in Figure 2a.Photoelectrodes with these materials can complete the entire PEC water-splitting reactions using only sunlight without external bias.However, they can absorb only ultraviolet light below 400 nm due to their band gaps above 3.0 eV, thereby utilizing the limited solar energy.Hence, PEC photoelectrodes based on these wide-band-gap semiconductors show a low solar-to-hydrogen (STH) efficiency below 3% (Figure 2b) [15].Therefore, semiconductors with a short band gap are necessary to utilize the sufficient solar energy for efficient PEC water splitting.
level (ECB) should be lower than the HER potential (0 versus reversible hydrogen electrode, RHE), while its VB energy level (EVB) should be higher than the OER potential (1.23 V versus RHE).ECB < 0 V versus RHE, EVB > 1.23 V versus RHE (4) Second, its band gap (Eg) should theoretically be larger than 1.23 eV because the difference between the HER potential and OER potential is 1.23 eV.Practically, an Eg of 1.5~1.8eV is necessary due to the HER/OER overpotentials (ηHER and ηOER) [9,10].
Among various semiconductors, few wide-band-gap semiconductors, such as TiO2 [11], ZnO [12], SrTiO3 [13], and Nb2O5 [14], meet these criteria, as shown in Figure 2a.Photoelectrodes with these materials can complete the entire PEC water-splitting reactions using only sunlight without external bias.However, they can absorb only ultraviolet light below 400 nm due to their band gaps above 3.0 eV, thereby utilizing the limited solar energy.Hence, PEC photoelectrodes based on these wide-band-gap semiconductors show a low solar-to-hydrogen (STH) efficiency below 3% (Figure 2b) [15].Therefore, semiconductors with a short band gap are necessary to utilize the sufficient solar energy for efficient PEC water splitting.[11][12][13][14][16][17][18].(b) Estimated STH efficiency and photocurrent from the band gap of semiconductors.Reprinted from [15] with permission from Springer Nature, copyright 2013.Intrinsic characteristics of short-band-gap semiconductors determine the charge transport direction.Figure 3 shows the energy levels of the semiconductor and electrolyte before and after contacting each other, depending on the intrinsic characteristics of the semiconductor.As illustrated in Figure 3, the downward band bending is formed when Nanomaterials 2023, 13, 3142 3 of 23 n-type semiconductors contact the electrolyte, while the upward band bending is formed when p-type semiconductors contact the electrolyte, due to the Fermi level equilibration between the semiconductor and electrolyte.Due to this band bending, holes in the n-type semiconductor easily move to the semiconductor/electrolyte interface, whereas electrons in the p-type semiconductor quickly move to the semiconductor/electrolyte interface.Therefore, in general, n-type semiconductors such as BiVO 4 [19][20][21], WO 3 [22][23][24], Fe 2 O 3 [25][26][27], and Ag 3 PO 4 [28] have been used as photoanodes for generating oxygen, while p-type semiconductors such as CuO [29][30][31], Cu 2 O [17,[32][33][34], Sb 2 Se 3 [35][36][37], LaFeO 3 [18,38,39], ternary copper oxide [40], and transitional metal dichalcogenides [41] have been used as photocathodes for generating hydrogen.
the band gap of semiconductors.Reprinted from [15] with permission from Springer Nature, copyright 2013.
Intrinsic characteristics of short-band-gap semiconductors determine the charge transport direction.Figure 3 shows the energy levels of the semiconductor and electrolyte before and after contacting each other, depending on the intrinsic characteristics of the semiconductor.As illustrated in Figure 3, the downward band bending is formed when n-type semiconductors contact the electrolyte, while the upward band bending is formed when p-type semiconductors contact the electrolyte, due to the Fermi level equilibration between the semiconductor and electrolyte.Due to this band bending, holes in the n-type semiconductor easily move to the semiconductor/electrolyte interface, whereas electrons in the p-type semiconductor quickly move to the semiconductor/electrolyte interface.Therefore, in general, n-type semiconductors such as BiVO4 [19][20][21], WO3 [22][23][24], Fe2O3 [25][26][27], and Ag3PO4 [28] have been used as photoanodes for generating oxygen, while ptype semiconductors such as CuO [29][30][31], Cu2O [17,[32][33][34], Sb2Se3 [35][36][37], LaFeO3 [18,38,39], ternary copper oxide [40], and transitional metal dichalcogenides [41] have been used as photocathodes for generating hydrogen.

Cuprous Oxide Photocathode
Cuprous oxide (Cu2O) is a remarkable photocathode material for the efficient and economical PEC water-splitting system.Its intrinsic characteristics and energy level are suitable for generating hydrogen via water reduction reactions at the interface.As shown in Figure 2, its ECB is much lower than the HER potential, which is advantageous for transporting electrons to the semiconductor/water interface.In addition, it is possible to utilize the visible light due to its band gap (2 eV).Theoretically, it enables Cu2O photocathodes to produce a high photocurrent density up to −14.7 mA cm −2 , corresponding to a solar-tohydrogen (STH) efficiency of 18% [15].Furthermore, it is cheap and earth-abundant.However, it is still challenging to achieve an efficient and durable PEC performance using only a Cu2O photoelectrode.Hence, the state-of-the-art Cu2O photocathode consists of a back and electrolyte before and after Fermi level equilibration (E F,equil ).E CB is an energy level of the conduction band, E VB is an energy level of the valence band, E F is the Fermi level, and E F,redox is a redox potential of the electrolyte.

Cuprous Oxide Photocathode
Cuprous oxide (Cu 2 O) is a remarkable photocathode material for the efficient and economical PEC water-splitting system.Its intrinsic characteristics and energy level are suitable for generating hydrogen via water reduction reactions at the interface.As shown in Figure 2, its E CB is much lower than the HER potential, which is advantageous for transporting electrons to the semiconductor/water interface.In addition, it is possible to utilize the visible light due to its band gap (2 eV).Theoretically, it enables Cu 2 O photocathodes to produce a high photocurrent density up to −14.7 mA cm −2 , corresponding to a solarto-hydrogen (STH) efficiency of 18% [15].Furthermore, it is cheap and earth-abundant.However, it is still challenging to achieve an efficient and durable PEC performance using only a Cu 2 O photoelectrode.Hence, the state-of-the-art Cu 2 O photocathode consists of a back contact layer, overlayer for the heterostructure, protection layer, and HER catalysts, as well as a Cu 2 O light absorber, as illustrated in Figure 4 [32,[42][43][44].
In this review paper, the fundamentals of each component in the Cu well as a Cu2O light absorber, as illustrated in Figure 4 [32,[42][43][44].
In this review paper, the fundamentals of each component in the Cu2O photocathode mentioned above are briefly introduced.In addition, recent research progress on these components for the efficient and durable Cu2O photocathode is summarized.Further, recent efforts in practical PEC water splitting based on Cu2O photocathodes are reviewed.Finally, the future outlook and research directions on the Cu2O photocathodes toward practical PEC water splitting are discussed.It will give comprehensive guidelines and insights on the efficient and durable Cu2O photocathode to researchers focusing on the practical PEC water-splitting system.

Cu2O Light Absorber
Cu2O is a key component of Cu2O photocathodes because it not only generates electron-hole pairs by absorbing sunlight but also transports the generated charges.Therefore, its optical and charge transport characteristics are a significant parameter affecting the PEC performance of Cu2O photocathodes.Based on the absorption coefficient, thick film above 1 µm is necessary to absorb sufficient light using the Cu2O light absorber [45][46][47].However, it generally has a limited minority carriers (electrons) diffusion length below 200 nm [45,48,49].In this regard, the generated electrons in the thick Cu2O film are easily recombined before reaching the water interface, irrespective of enough light utilization.On the other hand, the light utilization is not sufficient in the thin Cu2O film, even though it is advantageous for efficient electron transport.Therefore, it is necessary to overcome this mismatch for achieving the high PEC performance of Cu2O photocathodes.
The introduction of nanostructure is an effective strategy to enhance the light utilization by the light-trapping effect.Cu2O nanowire structures have been widely adopted in the Cu2O photocathode because of the simple fabrication process.In general, the metallic Cu substrate is transformed into the copper hydroxide (Cu(OH)2) nanowire electrode by the chemical reactions.Subsequently, it is converted into the Cu2O nanowire electrode via the thermal treatment.Hsu et al. fabricated Cu2O micro/nanostructured photocathodes by chemical oxidation and subsequent thermal reduction under a N2 atmosphere, as shown in Figure 5a [50].They found that the temperature of thermal reduction is crucial to obtain pure and well-structured Cu2O nanowire photocathodes.However, their devices had flower-like Cu2O structures with micro scales (2~3 µm) on Cu2O nanowires, thereby showing the limited PEC performance, corresponding to an STH efficiency of 1.97%.Salehmin et al. also demonstrated that the coverage of Cu2O microflowers on the Cu2O nanowires results in the reduced PEC performance because it interrupts the light penetration into the nanowire structure and it prolongs the electron transport length [51].They fabricated vertically aligned Cu2O nanowire photocathodes by controlling the ageing time of nanowire growth.It showed a better PEC performance, resulting from the improved charge

Fundamentals and Research Progress 2.1. Cu 2 O Light Absorber
Cu 2 O is a key component of Cu 2 O photocathodes because it not only generates electron-hole pairs by absorbing sunlight but also transports the generated charges.Therefore, its optical and charge transport characteristics are a significant parameter affecting the PEC performance of Cu 2 O photocathodes.Based on the absorption coefficient, thick film above 1 µm is necessary to absorb sufficient light using the Cu 2 O light absorber [45][46][47].However, it generally has a limited minority carriers (electrons) diffusion length below 200 nm [45,48,49].In this regard, the generated electrons in the thick Cu 2 O film are easily recombined before reaching the water interface, irrespective of enough light utilization.On the other hand, the light utilization is not sufficient in the thin Cu 2 O film, even though it is advantageous for efficient electron transport.Therefore, it is necessary to overcome this mismatch for achieving the high PEC performance of Cu 2 O photocathodes.
The introduction of nanostructure is an effective strategy to enhance the light utilization by the light-trapping effect.Cu 2 O nanowire structures have been widely adopted in the Cu 2 O photocathode because of the simple fabrication process.In general, the metallic Cu substrate is transformed into the copper hydroxide (Cu(OH)    [50] with permission from Elsevier Ltd., copyright 2013.(b) Cu2O microflowers/nanowires photocathode.Reprinted from [51] with permission from Elsevier Ltd., copyright 2018.(c) Cu2O nanowire photocathode fabricated by the anodization and annealing process.Reprinted with permission from [52].Copyright 2016 American Chemical Society.(d) Pyramidal Cu2O photocathode assisted with the pyramidal silicon template.Reprinted with permission from [53].Copyright 2022 American Chemical Society.(e) Cu2O micro pillar photocathode.Reprinted from [54] with permission from Elsevier B.V., copyright 2016.(f) Luo et al. developed high-quality Cu 2 O nanowire photocathodes with excellent electronic and photonic properties by an anodization process in a strong alkaline solution and an annealing process under an Ar atmosphere [52].The pure Cu 2 O nanowire photocathode was successfully completed using the Cu sputtered fluorine-doped tin oxide (FTO) substrate with a thickness of Cu above 1.5 µm.Such a thick Cu layer continuously provides the Cu source for the transformation of Cu(OH) 2 into pure Cu 2 O during the annealing process.In addition, they electrodeposited the Cu 2 O thin layer on the Cu 2 O nanowire photocathode as a blocking layer for preventing the charge recombination.As shown in Figure 5c, the improved incident-photon-to-current efficiency (IPCE) in the longwave length region (500~600 nm) was observed in their Cu 2 O nanowire photocathodes, due to the light-trapping effect by the nanowire structure.As a result, it showed a remarkably improved PEC performance compared to the planar Cu 2 O photocathodes.A current density was achieved up to −8 mA cm −2 at the HER potential, corresponding to an STH efficiency of 10%, combined with an aluminum-doped zinc oxide (AZO) overlayer and ruthenium oxide (RuO x ) HER catalysts.Interestingly, the diameter of the Cu 2 O nanowire in this literature is within the minority carrier diffusion length of Cu 2 O. Therefore, the nanowire structure is beneficial for better charge transport, as well as improved light utilization.
Recently, various template-assisted Cu 2 O nanostructure photocathodes have been also introduced.Zhao et al. developed pyramidal Cu 2 O photocathodes assisted with the pyramidal silicon (Si) template [53].It was fabricated by electrodepositing Cu 2 O film on the gold (Au) layered pyramidal Si template.The pyramidal structure exhibited a significantly reduced reflectance in the longwave length region above 500 nm (Figure 5d).This means that it is effective to enhance the light utilization by the light-trapping effect.Cu 2 O micro pillar photocathodes was successfully fabricated by Yoo's group [54].They used the nickel (Ni) micro pillars with heights of 16 µm and diameters of 1~1.5 µm as a template for synthesizing Cu 2 O micro pillar photocathodes.Contrary to other Cu 2 O nanowire photocathodes, it was aligned well perpendicularly to the substrate, as shown in Figure 5e.It provides more active sites for water reduction reactions, which is favorable to improve the PEC performance.Inverse-opal is a typical 3D nanostructure for improving the light utilization in photonic devices.Wu et al. recently developed Cu 2 O inverse-opal photocathodes using polystyrene (PS) microspheres as a colloidal template [55].The inverseopal Cu 2 O photocathode was successfully fabricated by removing the PS microsphere template after depositing a Cu 2 O film by the electrochemical deposition method.The light absorption characteristics of the inverse-opal Cu 2 O structure were dependent on the size of the PS microsphere template (Figure 5f).Based on this observation, they successfully optimized the inverse-opal structure for efficient Cu 2 O photocathodes.
The poor charge transport characteristics of Cu 2 O should also be improved for developing the efficient Cu 2 O photocathode.It is well-known that cation doping is generally effective to enhance the p-type conductivity of Cu 2 O electrodes, depending on the fabrication method [56][57][58].Hence, many groups have tried to apply various cations as a dopant to the Cu 2 O photocathode.The Ni-doped Cu 2 O photocathode is representative of these efforts [59,60].As illustrated in Figure 6a, in the Ni-doped Cu 2 O photocathode, the band gap became narrow due to an acceptor impurity from Ni dopants compared to the pristine Cu 2 O photocathode [60].It triggered not only accelerated charge separation but also extended light absorption.Furthermore, Ni dopants did not induce the structural distortion of the Cu 2 O photocathode because the ionic radius of Ni 2+ (0.72 Å) is similar to that of Cu + (0.77 Å).It promotes charge transport, thereby improving PEC performance.Alkaline ions (Li + , Na + , and K + ) were also used as a cation dopant by Chen et al. [61].Although these were all effective to improve charge transport, Li dopants were the most effective to improve the PEC performance (Figure 6b) because lithium(Li)-doped Cu 2 O photocathodes had less defects compared to Na-doped and K-doped Cu 2 O photocathodes.It is mainly due to the different ionic radii of alkaline ions: that of Li + (0.76 Å) is the closest to that of Cu + compared to others (Na + : 1.02 Å, and K + : 1.38 Å).Silver(Ag)-doped and iron(Fe)-doped Cu 2 O photocathodes were also introduced by Upadhyay et al. [62,63].They showed the improved PEC performances, compared to those of undoped Cu 2 O photocathodes (Figure 6c,d).It was mainly due to the enhanced charge transport and conductivity by the doping effect.However, the excessive cation doping had negative influences on the PEC performance because it caused a non-homogeneous Cu 2 O photocathode with many defect sites.Therefore, it is important to find the optimal doping level for the improved PEC performance of Cu 2 O photocathodes.
Nanomaterials 2023, 13, x FOR PEER REVIEW 7 of 23 photocathodes had less defects compared to Na-doped and K-doped Cu2O photocathodes.It is mainly due to the different ionic radii of alkaline ions: that of Li + (0.76 Å) is the closest to that of Cu + compared to others (Na + : 1.02 Å, and K + : 1.38 Å).Silver(Ag)-doped and iron(Fe)-doped Cu2O photocathodes were also introduced by Upadhyay et al. [62,63].They showed the improved PEC performances, compared to those of undoped Cu2O photocathodes (Figure 6c,d).It was mainly due to the enhanced charge transport and conductivity by the doping effect.However, the excessive cation doping had negative influences on the PEC performance because it caused a non-homogeneous Cu2O photocathode with many defect sites.Therefore, it is important to find the optimal doping level for the improved PEC performance of Cu2O photocathodes.The grain boundary of Cu2O photocathodes also affects the charge-transport characteristics.It is advantageous for the efficient Cu2O photocathode to alleviate the grain boundaries in the Cu2O because they act as a recombination center of charge, reducing the PEC performance.It is controllable to modify the fabrication process.Baek et al. introduced a thin antimony (Sb)-incorporated Cu2O (Cu2O:Sb) seed layer to grow the highlyoriented Cu2O photocathode using electrodeposition [64].The Cu2O:Sb seed layer was synthesized by electrodeposition in the Sb-ions-added copper sulfate aqueous solution.The Sb ions retard lateral diffusion of Cu ions in the solution, resulting in the verticallyoriented Cu2O seed layer.It facilitates the growth of highly-oriented Cu2O photocathode with less grain boundaries during the electrodeposition process (Figure 7a).Qin et al. recently compared the characteristics of electrodeposited Cu2O photocathodes and  [65].They demonstrated that the magnetron sputtering is more feasible to fabricate Cu 2 O photocathodes with less grain boundaries than electrodeposition (Figure 7b).These works reported the improved PEC performance of a Cu 2 O photocathode by the mitigation of grain boundaries in the Cu 2 O photocathode, as shown in Figure 7.
magnetron-sputtered Cu2O photocathodes [65].They demonstrated that the magnetron sputtering is more feasible to fabricate Cu2O photocathodes with less grain boundaries than electrodeposition (Figure 7b).These works reported the improved PEC performance of a Cu2O photocathode by the mitigation of grain boundaries in the Cu2O photocathode, as shown in Figure 7.

Back Contact Layer
The back contact layer is responsible for forwarding generated holes in the Cu2O layer to the external load via the conductive substrate.In general, the thin metal film is employed as a back contact layer; thus, the Cu2O/metal interface is critical to promote the migration of holes.The characteristic of the Cu2O/metal interface is entirely dependent on the work function of Cu2O (φCu2O) and metal (φmetal).When the φmetal is smaller than φCu2O, Schottky contact is formed by the Fermi level equilibration, thereby preventing the hole transportation into the conductive substrate (Figure 8a).On the other hand, the φmetal is larger than φCu2O, Ohmic contact is formed by the Fermi level equilibration, facilitating the accelerated migration of holes to the conductive substrate (Figure 8b).Hence, the metal should have a larger work function than that of Cu2O (4.84 eV) [66] for fitting into the back contact layer for the Cu2O photocathode.Table 1 shows metals with a large work function.These are available as a back contact layer for Cu2O photocathodes because their work functions are larger than that of Cu2O, leading to the Ohmic contact for hole migrations.

Back Contact Layer
The back contact layer is responsible for forwarding generated holes in the Cu 2 O layer to the external load via the conductive substrate.In general, the thin metal film is employed as a back contact layer; thus, the Cu 2 O/metal interface is critical to promote the migration of holes.The characteristic of the Cu 2 O/metal interface is entirely dependent on the work function of Cu 2 O (ϕ Cu 2 O ) and metal (ϕ metal ).When the ϕ metal is smaller than ϕ Cu 2 O , Schottky contact is formed by the Fermi level equilibration, thereby preventing the hole transportation into the conductive substrate (Figure 8a).On the other hand, the ϕ metal is larger than ϕ Cu 2 O , Ohmic contact is formed by the Fermi level equilibration, facilitating the accelerated migration of holes to the conductive substrate (Figure 8b).Hence, the metal should have a larger work function than that of Cu 2 O (4.84 eV) [66] for fitting into the back contact layer for the Cu 2 O photocathode.Table 1 shows metals with a large work function.These are available as a back contact layer for Cu 2 O photocathodes because their work functions are larger than that of Cu 2 O, leading to the Ohmic contact for hole migrations.A Au back contact layer has been frequently used for Cu 2 O photocathodes [17,32,33,70].The Cu 2 O photocathode with a Au back contact layer shows a reliable and outstanding PEC performance, due to its excellent hole collection characteristic.Nevertheless, it is disadvantageous for the large-scale application and tandem configuration with a shortband-gap semiconductor, because it is expensive and opaque, respectively.To solve this limitation, Dias et al. developed transparent Cu 2 O photocathodes with a thin Au back contact layer [17].They found that a thin Au layer below 5 nm is sufficient for collecting holes, similar to a thick Au layer.It also enables the usage of Au to be reduced.The Cu back contact layer is a promising candidate for replacing the expensive Au back contact layer because it is earth-abundant and low-cost [52,71].Cu 2 O photocathodes with a Cu back contact layer generally showed similar PEC performances with Cu 2 O photocathodes with a Au back contact layer.A Au back contact layer has been frequently used for Cu2O photocathodes [17,32,33,70].The Cu2O photocathode with a Au back contact layer shows a reliable and outstanding PEC performance, due to its excellent hole collection characteristic.Nevertheless, it is disadvantageous for the large-scale application and tandem configuration with a short-band-gap semiconductor, because it is expensive and opaque, respectively.To solve this limitation, Dias et al. developed transparent Cu2O photocathodes with a thin Au back contact layer [17].They found that a thin Au layer below 5 nm is sufficient for collecting holes, similar to a thick Au layer.It also enables the usage of Au to be reduced.The Cu back contact layer is a promising candidate for replacing the expensive Au back contact layer because it is earth-abundant and low-cost [52,71].Cu2O photocathodes with a Cu back contact layer generally showed similar PEC performances with Cu2O photocathodes with a Au back contact layer.
Non-metallic materials have been explored as an alternative to the metallic back contact layer for Cu2O photocathodes.The main goal for these efforts is to develop the affordable Cu2O photocathode avoiding the usage of precious components.Nickel oxide (NiO) is a well-known material as a hole selective layer in perovskite solar cells [72][73][74].It is also possible to utilize the back contact layer of Cu2O photocathodes, because it has a larger work function (5.0 eV) than that of Cu2O.As shown in Figure 9a, it forms the Ohmic contact with Cu2O for smoothly migrating holes, while the fluorine-doped tin oxide (FTO) plied as a back contact layer of a Cu2O photocathode in the work by Pan et al. [77].Alt-hough the hole transport from Cu2O into CuSCN is difficult due to the VB offset in terms of energy level, solution-processed CuSCN thin film facilitates the smooth hole transport from Cu2O into conductive substrates by the band-tail states existence, as illustrated in Figure 9c.Moreover, the huge barrier generated by the large CB band offset effectively prevents the charge recombination at the back contact interface.Zhou et al. suggested iron oxide hydroxide (FeOOH) as a hole transfer layer in the Cu2O photocathode [78].Electrodeposited FeOOH thin film promotes the extraction of holes from Cu2O into conductive substrates due to its energy level (Figure 9d).Hence, the Cu2O photocathodes with the FeOOH back contact layer showed not only an enhanced PEC performance but also an improved stability.

Overlayer
A heterostructured Cu2O photocathode with a semiconductor overlayer is a promising strategy to improve the overall water-splitting performance of Cu2O photocathodes because the semiconductor overlayer renders the accelerated electrons/holes' separation and promotes electron transport from Cu2O into the water interface.However, the mechanism is slightly different, depending on the intrinsic characteristics of the semiconductor overlayer (n-type or p-type).When the p-n heterostructure is formed with an n-type semiconductor overlayer in the Cu2O photocathode, the built-in voltage (VBI) created from the

Overlayer
A heterostructured Cu 2 O photocathode with a semiconductor overlayer is a promising strategy to improve the overall water-splitting performance of Cu 2 O photocathodes because the semiconductor overlayer renders the accelerated electrons/holes' separation and promotes electron transport from Cu 2 O into the water interface.However, the mechanism is slightly different, depending on the intrinsic characteristics of the semiconductor overlayer (n-type or p-type).When the p-n heterostructure is formed with an n-type semiconductor overlayer in the Cu 2 O photocathode, the built-in voltage (V BI ) created from the difference of E F and E F,redox further increases compared to the single Cu 2 O photocathode (Figure 10a,b) [79].It promotes the electrons/holes separation and transport.On the other hand, the staircase-type energy levels are created, when the p-p heterostructure is formed with p-type semiconductor overlayer, which has a relatively lower CB and VB than Cu 2 O (Figure 10c) [80].It accelerates the electron transport into the water interface.Therefore, the n-type semiconductor-overlayered Cu 2 O photocathodes show a remarkable early onset potential, as well as an improved photocurrent density, whereas the p-type semiconductoroverlayered Cu 2 O photocathodes primarily show an improved photocurrent density.
Table 2 summarizes the onset potential and photocurrent density at the HER potential of heterostructured Cu 2 O photocathodes with n-type or p-type semiconductor overlayers.In the case of p-n heterostructures, Cu 2 O photocathodes with a TiO 2 overlayer show an improved onset potential (earlier onset potential) compared to the single Cu 2 O photocathode.The Cu 2 O/TiO 2 heterojunction reinforces the band bending, resulting in an improved PEC performance [81,82].However, TiO 2 has been widely applied to the protection layer of Cu 2 O photocathodes, rather than the overlayer for the heterojunction effect, because it is an intrinsically stable oxide in the aqueous solution.Although the ZnO overlayer is also effective to improve the charge transport by the formation of heterojunction with Cu 2 O, the aluminum-doped zinc oxide (AZO) overlayer is more efficient for the heterojunction effect because it is more conductive compared to the ZnO overlayer [83,84].Minami [87].Their devices also showed the improved onset potential, approximately 1.0 V versus RHE.It is advantageous for improving the PEC performance of an unbiased water-splitting system with the Cu 2 O photocathode and photoanode/photovoltaic [88].Consequently, they succeeded in demonstrating the unbiased all-oxide solar water-splitting system using a Ga 2 O 3 heterostructured Cu 2 O photocathode and transparent BiVO 4 photoanode with an operating current density of 2.5 mA cm −1 , corresponding to an STH efficiency of 3%.On the contrary to the n-type semiconductor overlayers, most researchers have used a cupric oxide (CuO) overlayer to form the p-p heterostructure for the Cu 2 O photocathode because it is easily prepared using a pure Cu 2 O photocathode via a thermal oxidation process [89][90][91][92].The electrons are easily moved to the water interface by the staircase-type energy level due to the energy levels of CuO and Cu 2 O, as illustrated in Figure 10c.In addition, the light utilization is also enhanced in the CuO/Cu 2 O heterostructured photocathode, due to a narrow band gap of CuO (1.3~1.7 eV) [93,94].Hence, the improvement of photocurrent density is prominent, rather than the enhancement of onset potential, in the CuO-overlayered Cu  [79].It promotes the electrons/holes separation and transport.On the other hand, the staircase-type energy levels are created, when the p-p heterostructure is formed with p-type semiconductor overlayer, which has a relatively lower CB and VB than Cu2O (Figure 10c) [80].It accelerates the electron transport into the water interface.Therefore, the n-type semiconductor-overlayered Cu2O photocathodes show a remarkable early onset potential, as well as an improved photocurrent density, whereas the p-type semiconductor-overlayered Cu2O photocathodes primarily show an improved photocurrent density.Table 2 summarizes the onset potential and photocurrent density at the HER potential of heterostructured Cu2O photocathodes with n-type or p-type semiconductor overlayers.In the case of p-n heterostructures, Cu2O photocathodes with a TiO2 overlayer show an improved onset potential (earlier onset potential) compared to the single Cu2O photocathode.The Cu2O/TiO2 heterojunction reinforces the band bending, resulting in an improved PEC performance [81,82].However, TiO2 has been widely applied to the protection layer of Cu2O photocathodes, rather than the overlayer for the heterojunction effect, because it is an intrinsically stable oxide in the aqueous solution.Although the ZnO overlayer is also effective to improve the charge transport by the formation of heterojunction with Cu2O, the aluminum-doped zinc oxide (AZO) overlayer is more efficient for the heterojunction effect because it is more conductive compared to the ZnO overlayer [83,84].Minami et al. reported that the gallium oxide (Ga2O3)/Cu2O heterostructure improves the photovoltage of Cu2O-based solar cells due to the decreased defect levels at the interfaces [85].Inspired by this work, Li et al. introduced the Ga2O3 overlayer in the Cu2O photocathode for improving the PEC performance [86].The improved photovoltage by the Ga2O3/Cu2O heterostructure leads to a remarkable enhanced onset potential in the Cu2O photocathodes.Pan et al. further improved the PEC performance of Ga2O3 overlayered Cu2O photocathodes with the Cu2O nanowire and ruthenium oxide (RuOx) HER catalysts [87].Their devices also showed the improved onset potential, approximately 1.0 V versus RHE.It is advantageous for improving the PEC performance of an unbiased water-splitting system with the Cu2O photocathode and photoanode/photovoltaic [88].Consequently, they succeeded in demonstrating the unbiased all-oxide solar water-splitting system using a Ga2O3 heterostructured Cu2O photocathode and transparent BiVO4 photoanode with an operating current density of 2.5 mA cm −1 , corresponding to an STH efficiency of 3%.
On the contrary to the n-type semiconductor overlayers, most researchers have used a cupric oxide (CuO) overlayer to form the p-p heterostructure for the Cu2O photocathode because it is easily prepared using a pure Cu2O photocathode via a thermal oxidation process [89][90][91][92].The electrons are easily moved to the water interface by the staircase-type energy level due to the energy levels of CuO and Cu2O, as illustrated in Figure 10c.In addition, the light utilization is also enhanced in the CuO/Cu2O heterostructured photocathode, due to a narrow band gap of CuO (1.3~1.7 eV) [93,94].Hence, the improvement of photocurrent density is prominent, rather than the enhancement of onset potential, in

Protection Layer
The main challenge of the Cu 2 O photocathode is its poor stability against water.It is easily degraded in the aqueous solution within a few minutes, thereby losing its PEC characteristics [32].Figure 11 illustrates the stability change in the semiconductor in the water.In general, the semiconductor is readily oxidized in the water when its oxidation potential is smaller than the OER potential, while it is simply reduced in the water when its reduction potential is larger than the HER potential, instead of aiding in water-splitting reactions [95].Cu 2 O meets these two conditions: It is reduced into the metallic Cu in the potential window of 0.3~0.4V versus RHE, whereas it is oxidized into CuO or copper hydroxide (Cu(OH) 2 ) in the potential window of 0.6~1.05V versus RHE (Figure 11) [96].Hence, it is extremely unstable in water.This is a reason why the protection layer is an essential component in the Cu 2 O photocathode for durable PEC water splitting.Many researchers have used a TiO2 thin film as a protection layer for the Cu2O photocathode because it is intrinsically stable in water.Its reduction potential is larger than the HER potential; thus, it is robust to corrosion in the aqueous solution.In addition, its CB is beneficial for transporting electrons into the water interface [95].The TiO2 protection layer is generally deposited on the Cu2O photocathodes by atomic layer deposition (ALD), which is favorable to deposit the homogeneous thin layer with the thickness of nanometer Many researchers have used a TiO 2 thin film as a protection layer for the Cu 2 O photocathode because it is intrinsically stable in water.Its reduction potential is larger than the HER potential; thus, it is robust to corrosion in the aqueous solution.In addition, its CB is beneficial for transporting electrons into the water interface [95].The TiO 2 protection layer is generally deposited on the Cu 2 O photocathodes by atomic layer deposition (ALD), which is favorable to deposit the homogeneous thin layer with the thickness of nanometer scale.The benchmark stability of Cu 2 O photocathodes was recorded by Prof. Grätzel's group using an ALD-deposited amorphous TiO 2 protection layer in 2018 [87].Their devices showed a remarkable stability beyond 100 h, as shown in Figure 12a.However, it was gradually degraded after PEC operation for 100 h.This means that the amorphous TiO 2 protection layer is not sufficient for completely protecting the Cu 2 O photocathode.
Many efforts have been made to improve its protection capability.Azevedo et al. carried out low-temperature steam treatment on the amorphous TiO 2 -protected Cu 2 O photocathode [97].Although the crystallinity of TiO 2 was not changed, the surface of the TiO 2 became much smoother after the steam treatment (Figure 12b).It cured any defects and cracks in the TiO 2 protection layer, resulting in an excellent durability with 10% of PEC performance loss over more than 50 h.The crystallization of the amorphous TiO 2 protection layer is also one of these efforts because the crystalline TiO 2 is more robust than the amorphous one [96,98].Nishikawa et al. succeeded in applying the crystalline TiO 2 protection layer on the Cu 2 O photocathodes using the solution process assisted with excimer laser irradiation [99].As shown in Figure 12c, the mild laser irradiation with continuous shots induced the crystallization of TiO 2 , especially the rutile phase.It was demonstrated that it efficiently prevents the redox reaction of Cu 2 O, facilitating the stability enhancement of Cu 2 O photocathodes.Furthermore, the structural modification of the TiO 2 protective layer enables the long-term stability of the Cu 2 O photocathode to improve.A thick TiO 2 protection layer above 100 nm is beneficial to improve the stability, but it is prone to show a decreased PEC performance due to the disturbance of electron transport.Kim et al. solved this mismatch by introducing a metallic nano filament to reinforce the electron transport in the thick TiO 2 protection layer [100].Consequently, the Cu 2 O photocathode showed an excellent stability with a considerable PEC performance for 100 h (Figure 12d).
A conductive layer also assists to enhance the long-term stability of Cu 2 O photocathodes via the fast electron transfer from the Cu 2 O into the water interface.In this regard, some groups used the metallic layer, such as Au and Ag, for protecting the degradation of Cu 2 O photocathodes [101,102].Its thickness should be considered because it can block the light irradiation to the Cu 2 O because of its opacity, leading to the decreased PEC performance.A thin metallic layer with tens of nanometer is advantageous to improve the stability without decreasing the PEC performance.Carbon-based material has been explored as a conductive protection layer for Cu 2 O photocathodes.Kunturu et al. developed CuO heterostructured Cu 2 O photocathodes with a 15 nm carbon layer [103].The thin carbon layer suppresses the photocorrosion of Cu 2 O photocathodes by facilitating fast electron transfer to the surface.Das et al. tried to use a graphene layer, which is more conductive than pure carbon, as a protection layer [104].They deposited the graphene protective layer on the Cu 2 O photocathode by the chemical vapor deposition (CVD) method.The CVDfabricated graphene layer had several microcracks, resulting in the degradation of Cu 2 O.They solved this shortcoming by introducing a thin TiO 2 layer below 10 nm.It patches the microcrack in the graphene layer, thereby efficiently inhibiting the photocorrosion of Cu 2 O. Titanium nitride (TiN) was also used as a protection layer in the work by Diao et al., because it is highly conductive and corrosion-resistant [105].They controlled the thickness of TiN by adjusting the cycle of ALD deposition to trade off the PEC performance and the stability.Finally, the Cu 2 O photocathodes with the ultra-thin TiN protection layer (8 nm) showed 100% stability during the PEC operation for 1 hr, without critical deterioration of the PEC performance.rosion of Cu2O.Titanium nitride (TiN) was also used as a protection layer in the work by Diao et al., because it is highly conductive and corrosion-resistant [105].They controlled the thickness of TiN by adjusting the cycle of ALD deposition to trade off the PEC performance and the stability.Finally, the Cu2O photocathodes with the ultra-thin TiN protection layer (8 nm) showed 100% stability during the PEC operation for 1 hr, without critical deterioration of the PEC performance.Recently, organic materials have also received a lot of attention as a protective layer for Cu2O photocathodes.Li et al. introduced the compact polymer layer for protecting Cu2O photocathodes [106].Polyurethane acrylate (PUA) thin film was used to cover the Cu2O by a solution process using a viscous urethane acrylate monomer.Their device showed a 98% photo-durability during PEC operation under continuous light Recently, organic materials have also received a lot of attention as a protective layer for Cu 2 O photocathodes.Li et al. introduced the compact polymer layer for protecting Cu 2 O photocathodes [106].Polyurethane acrylate (PUA) thin film was used to cover the Cu 2 O by a solution process using a viscous urethane acrylate monomer.Their device showed a 98% photo-durability during PEC operation under continuous light illumination for 6 h.Zhang et al. developed phenylethynyl copper (Ph-C≡C-Cu)-protected Cu 2 O photocathodes [107].The Ph-C≡C-Cu layer was a self-assembled monolayer fabricated by a photothermal method.Hence, the conformal Ph-C≡C-Cu protection layer was successfully applied to the Cu 2 O photocathode.As a result, they constructed a stable unbiased water-splitting system operating for 5 h using the Ph-C≡C-Cu-protected Cu 2 O photocathode with the NiOOH/FeOOH-layered BiVO 4 photoanode.These protection layers based on organic materials have hydrophobic characteristics, which can inhibit the corrosion of Cu 2 O by avoiding contact with the aqueous solution.Hence, it is thoroughly effective to prolong the stability of Cu 2 O photocathodes.

Co-Catalysts
In general, HER catalysts have been employed as a co-catalyst on the surface of the protection layer of Cu 2 O photocathodes to reduce the overpotential of the water reduction reaction, leading to the improved PEC performance of Cu 2 O photocathodes.It is well-known that platinum (Pt) is the most promising HER catalyst because of its highly intrinsic activity in the water reduction reaction and low hydrogen adsorption energy [108,109].Thus, it has been actively adopted for improving the PEC performance of Cu 2 O photocathodes [32,43,[110][111][112][113].Nevertheless, it is unfavorable for long-term stability because it detaches from the Cu 2 O photocathodes during PEC operation.It is mainly attributed to the weak bonding of the Pt and TiO 2 protection layer [96].RuO x , which was suggested by Tilley et al. [114], is an alternative HER catalyst to Pt for the durable PEC operation of Cu 2 O photocathodes.It is more robust than Pt due to the strong bonding with the TiO 2 protection layer.Hence, the RuO x -photoelectrodeposited Cu 2 O photocathode showed a better stability with a competitive PEC performance, compared to the Pt-catalyzed Cu 2 O photocathode.However, these components (Pt and RuO x ) are not suitable for the economical PEC water-splitting system because they are a noble metal.Therefore, many researchers have pursued the application of inexpensive HER catalysts with low-cost materials to the Cu 2 O photocathodes for obtaining the economic feasibility of the PEC water-splitting system.
Ni-based materials are a promising candidate as a low-cost HER catalyst.Lin et al. deposited the NiO/nickel hydroxide (Ni(OH) 2 ) composite on the Cu 2 O nanowire photocathode by the solution process and sequential annealing process [115].They found that the NiO/Ni(OH) 2 composites allow for an improved charge transfer by retarding charge recombination via the synergy effect of NiO and Ni(OH) 2 .Hence, the NiO/Ni(OH) 2 composite-decorated Cu 2 O nanowire photocathode showed an enhanced PEC performance.Recently, Jian et al. developed a highly efficient NiO x composite (mixture of Ni and NiO) HER catalyst fabricated by vacuum evaporation deposition and heat treatment [116].As shown in Figure 13a, it showed a remarkable HER activity (Tafel slope of 35.9 mV dec −1 ), which was very close to the HER activity of Pt (Tafel slope of 32.5 mV dec −1 ).Thus, the NiO x -catalyzed Cu 2 O photocathode showed a positive shift of onset potential from 0.2 V to 0.6 V versus RHE.Furthermore, it showed quite a stable PEC performance for 90 min.This means that the NiO x composite plays a role as not only an HER catalyst but also as a protection layer for Cu 2 O photocathodes.The electrodeposited Ni-Mo (Molybdenum) alloy, which was suggested in the work by Morales-Guio et al. [117], also significantly reduced the HER overpotential in an alkaline aqueous solution.The Ni-Mo-decorated Cu 2 O photocathode produced a high photocurrent density of −6.3 mA cm −2 at 0 V versus RHE without any dark currents in the basic solution (Figure 13b).Qi et al. successfully applied Ni-Fe (Iron)-layered double hydroxide (LDH) to the Cu 2 O photocathode [118].The electrodeposited Ni-Fe LDH catalysts boosted the electron transfer into the water interface by the appropriate band alignment with Cu 2 O. Consequently, the Ni-Fe LDH co-catalyzed Cu 2 O photocathode showed a seven-fold increase in PEC performance with remarkable stability for 40 h compared to a bare Cu 2 O photocathode (Figure 13c).
Transition metal sulfides have been considered as a good HER catalyst for Cu 2 O photocathodes, due to their high conductivities.Morales-Guio et al. deposited molybdenum sulfide (MoS x ) co-catalysts on the TiO 2 -protected Cu 2 O photocathode by photoelectrodeposition [119].The MoS x -loaded Cu 2 O photocathode showed an outstanding PEC performance with a photocurrent density of −5.7 mA cm −2 at the HER potential, corresponding to an STH efficiency of 7%, in the strong acidic electrolyte (pH 1), as shown in Figure 14a.Although the condition of the electrolyte was extremely harsh, their devices survived with a reliable PEC performance for 10 h.Chen et al. developed nickel sulfide (NiS)combining aluminum (Al) nanoparticles HER catalysts for the Cu 2 O photocathode [120].NiS HER catalysts were deposited by a successive ionic layer adsorption reaction (SILAR) method.They demonstrated that the NiS attracts more protons (H + ) into the surface of Cu 2 O photocathodes and forms enriched electron conditions by the Ni-H bonds, resulting in a reduced HER overpotential.As a result, the Al nanoparticles NiS-co-catalyzed Cu 2 O photocathode showed an improved PEC performance, along with the plasmonic effect by the Al nanoparticles (Figure 14b).Transition metal sulfides have been considered as a good HER catalyst for Cu2O photocathodes, due to their high conductivities.Morales-Guio et al. deposited molybdenum sulfide (MoSx) co-catalysts on the TiO2-protected Cu2O photocathode by photoelectrodeposition [119].The MoSx-loaded Cu2O photocathode showed an outstanding PEC performance with a photocurrent density of −5.7 mAcm −2 at the HER potential, corresponding to an STH efficiency of 7%, in the strong acidic electrolyte (pH 1), as shown in Figure 14a.Although the condition of the electrolyte was extremely harsh, their devices survived with a reliable PEC performance for 10 h.Chen et al. developed nickel sulfide (NiS)-combining aluminum (Al) nanoparticles HER catalysts for the Cu2O photocathode [120].NiS HER catalysts were deposited by a successive ionic layer adsorption reaction (SILAR) method.They demonstrated that the NiS attracts more protons (H + ) into the surface of Cu2O photocathodes and forms enriched electron conditions by the Ni-H bonds, resulting in a reduced HER overpotential.As a result, the Al nanoparticles NiS-co-catalyzed Cu2O photocathode showed an improved PEC performance, along with the plasmonic effect by the Al nanoparticles (Figure 14b).Transition metal phosphides have also been explored as low-cost HER catalysts for Cu 2 O photocathodes because they have an excellent catalytic activity for HER.Cobalt phosphide (CoP) catalysts were successfully adopted for the Cu 2 O photocathode in the work by Stern et al. [121].Nanoflower-like CoP catalysts showed an extremely low HER overpotential of approximately 97 mV in a strong acidic condition to reach a current density of 10 mA cm −2 .Consequently, their CoP-decorated Cu 2 O photocathodes showed a considerable photocurrent density of −5.3 mA cm −2 at the HER potential in the strong acidic electrolyte, which is a favorable condition for the water reduction reaction (Figure 14c).Chhetri et al. developed nickel phosphide (NiP)-decorated Cu 2 O photocathodes [122].They successfully deposited NiP catalysts on the CuO heterostructured Cu 2 O photocathode by the pulse plating electrodeposition.Their devices showed a noticeably improved PEC performance by the fast electron transfer into the water interface due to the NiP catalysts compared to the bare Cu 2 O photocathodes (Figure 14d).

Outlook and Future Research Directions
Four aspects should be considered for the practical PEC water-splitting system: efficiency, stability, cost, and mass production.In terms of efficiency and cost, Cu 2 O is a frontrunner photocathode material for the practical PEC water-splitting system, because the state-of-the-art Cu 2 O photocathode shows a high STH efficiency above 10%, which is a benchmark for the commercialization.In addition, it is an earth-abundant material.However, it is necessary to further improve its stability above 100 h for the durable PEC water-splitting operation.Furthermore, upscaling is essential for the mass production of hydrogen via the PEC water-splitting system based on Cu 2 O photocathodes.Therefore, future research on the Cu 2 O photocathode should move forward based on strategies as below.

1.
Cu 2 O absorber: The further improvement of electron transport capability will be necessary.To this end, doping is the most efficient strategy.In the case of this strategy, the ionic radius of the dopant should be similar to that of Cu + ions for reducing the defects on the Cu 2 O film.In addition, the doping level should be optimized for improving the electron transport in the Cu 2 O photocathodes, because the excessive doping has a negative influence on the PEC performance.The fabrication of a high-quality Cu 2 O film with less defects or grain boundaries is also advantageous to improve the electron transport in the Cu 2 O photocathode.Furthermore, the development of transparent Cu 2 O photocathodes with efficient PEC performance paves the way for developing the efficient PEC-PEC or PEC-PV water-splitting system with short-band-gap materials; 2.
Back contact layer: The development of an alternative back contact layer to the expensive Au back contact layer is a main goal of this component.In the case of metal, its work function should be higher than that of Cu 2 O.Moreover, control of the opacity is necessary for the development of transparent Cu 2 O photocathodes.
A semiconductor with a huge energy barrier is a good option because it efficiently hinders the electron recombination at the interface.In this case, the suitable deposition method of Cu 2 O should be considered on the semiconductor-based back contact layer; 3.
Overlayer: In the case of n-type overlayers, the created photovoltage in contact with Cu 2 O should be considered because it motivates the charge separation in the p-n junction with Cu 2 O.In the case of p-type overlayers, the exploration on the alternative material to the CuO overlayer with the proper energy level for enhancing the electron transfer into the water interface is a good strategy for the future research direction; 4.
Protection layer: Although the amorphous TiO 2 protection layer is highly efficient for protecting a Cu 2 O photocathode against the corrosion, it is still not sufficient due to its pinholes or defects.Hence, the reduction in pinholes or defect of the amorphous TiO 2 protection layer is useful for further improvement of its protection capability.The crystallization method of the TiO 2 protection layer without damaging the Cu 2 O photocathode is also feasible to improve the stability of the Cu 2 O photocathode without a decreased PEC performance.The technique to form the hydrophobic surface on the Cu 2 O photocathode is a promising strategy to improve the stability of Cu 2 O photocathodes; 5.
Co-catalysts: The development of non-noble HER catalysts and the alleviation of noble components in HER catalysts is essential for the low-cost PEC water-splitting system.Although various HER catalysts have recently been developed [123], the deposition method should be considered for successfully applying to the Cu 2 O photocathode.Furthermore, the bonding of the HER catalyst with a Cu 2 O photocathode should be concerned for the durable Cu 2 O photocathode because it is directly related to the stability; 6.
Upscaling: The reported high PEC performance of Cu 2 O photocathodes is normally based on a small scale below 1 cm 2 .In general, it is significantly reduced in the large-scale Cu 2 O photocathodes [124].Therefore, the research on maintaining its high PEC performance in the large-scale Cu 2 O photocathodes is necessary, such as a novel design.Although a few groups have recently reported their works on the large-scale Cu 2 O photocathode [124,125], more vigorous efforts on this are still essential for realizing the mass production of hydrogen via the PEC water-splitting system based on the Cu 2 O photocathode in the future.

Conclusions
Cu 2 O photocathodes have rapidly advanced for the practical PEC water-splitting system.Although some challenges remain to be overcome, such as the stability and the upscaling, the commercialization of the PEC water-splitting system using Cu 2 O photocathodes is remarkably optimistic in the future, due to its potential for a high PEC performance and economic feasibility.Key strategies on each component in the Cu 2 O photocathode, as suggested in this review paper, provide a shortcut for realizing this optimistic prospect.Furthermore, it will be the cornerstone of the successful entrance of a practical PEC watersplitting system into the eco-friendly hydrogen-fuel-based economy in the near future.

Figure 2 .
Figure 2. (a) Band-gap information of typical PEC semiconductors.It is drawn based on the information in the reported literature [11-14,16-18].(b) Estimated STH efficiency and photocurrent from

Figure 1 .
Figure 1.Operational principle of PEC water splitting with two semiconductor electrodes.

Figure 2 .
Figure 2. (a) Band-gap information of typical PEC semiconductors.It is drawn based on the information in the reported literature [11-14,16-18].(b) Estimated STH efficiency and photocurrent from Figure 2. (a) Band-gap information of typical PEC semiconductors.It is drawn based on the information in the reported literature [11-14,16-18].(b) Estimated STH efficiency and photocurrent from the band gap of semiconductors.Reprinted from [15] with permission from Springer Nature, copyright 2013.

Figure 3 .
Figure 3. Energy levels of the semiconductor ((a) n-type semiconductor and (b) p-type semiconductor) and electrolyte before and after Fermi level equilibration (EF,equil).ECB is an energy level of the conduction band, EVB is an energy level of the valence band, EF is the Fermi level, and EF,redox is a redox potential of the electrolyte.

Figure 3 .
Figure 3. Energy levels of the semiconductor ((a) n-type semiconductor and (b) p-type semiconductor)and electrolyte before and after Fermi level equilibration (E F,equil ).E CB is an energy level of the conduction band, E VB is an energy level of the valence band, E F is the Fermi level, and E F,redox is a redox potential of the electrolyte.
2 O photocathode mentioned above are briefly introduced.In addition, recent research progress on these components for the efficient and durable Cu 2 O photocathode is summarized.Further, recent efforts in practical PEC water splitting based on Cu 2 O photocathodes are reviewed.Finally, the future outlook and research directions on the Cu 2 O photocathodes toward practical PEC water splitting are discussed.It will give comprehensive guidelines and insights on the efficient and durable Cu 2 O photocathode to researchers focusing on the practical PEC water-splitting system.

Figure 4 .
Figure 4. Schematic structure of the state-of-the-art Cu2O photocathode.

Figure 4 .
Figure 4. Schematic structure of the state-of-the-art Cu 2 O photocathode.
2 ) nanowire electrode by the chemical reactions.Subsequently, it is converted into the Cu 2 O nanowire electrode via the thermal treatment.Hsu et al. fabricated Cu 2 O micro/nanostructured photocathodes by chemical oxidation and subsequent thermal reduction under a N 2 atmosphere, as shown in Figure 5a [50].They found that the temperature of thermal reduction is crucial to obtain pure and well-structured Cu 2 O nanowire photocathodes.However, their devices had flower-like Cu 2 O structures with micro scales (2~3 µm) on Cu 2 O nanowires, thereby showing the limited PEC performance, corresponding to an STH efficiency of 1.97%.Salehmin et al. also demonstrated that the coverage of Cu 2 O microflowers on the Cu 2 O nanowires results in the reduced PEC performance because it interrupts the light penetration into the nanowire structure and it prolongs the electron transport length [51].They fabricated vertically aligned Cu 2 O nanowire photocathodes by controlling the ageing time of nanowire growth.It showed a better PEC performance, resulting from the improved charge transport and prolonged light penetration path, compared to one in the work by Hsu et al. [50] (Figure 5b).
transport and prolonged light penetration path, compared to one in the work by Hsu et al.[50] (Figure5b).

Figure 6 .
Figure 6.Cation-doped Cu 2 O photocathodes.(a) Ni-doped Cu 2 O photocathode fabricated by a one-pot hydrothermal method.Reprinted from [60] with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, copyright 2020.(b) PEC performances of an alkaline ion (Li + , Na + , and K + )-doped Cu 2 O photocathode.Reprinted from [61] with permission from Hydrogen Energy Publications LLC, Elsevier Ltd., copyright 2018.(c) PEC performances of a Ag-doped Cu 2 O photocathode.Reprinted from [62] with permission from Springer Nature, copyright 2013.(d) PEC performances of an Fe-doped Cu 2 O photocathode.Reprinted from [63] with permission from Elsevier B.V., copyright 2015.The grain boundary of Cu 2 O photocathodes also affects the charge-transport characteristics.It is advantageous for the efficient Cu 2 O photocathode to alleviate the grain boundaries in the Cu 2 O because they act as a recombination center of charge, reducing the PEC performance.It is controllable to modify the fabrication process.Baek et al. introduced a thin antimony (Sb)-incorporated Cu 2 O (Cu 2 O:Sb) seed layer to grow the highly-oriented Cu 2 O photocathode using electrodeposition [64].The Cu 2 O:Sb seed layer was synthesized by electrodeposition in the Sb-ions-added copper sulfate aqueous solution.The Sb ions retard lateral diffusion of Cu ions in the solution, resulting in the vertically-oriented Cu 2 O seed layer.It facilitates the growth of highly-oriented Cu 2 O photocathode with less grain boundaries during the electrodeposition process (Figure 7a).Qin et al. recently compared the characteristics of electrodeposited Cu 2 O photocathodes and magnetron-sputtered Cu 2 O photocathodes[65].They demonstrated that the magnetron sputtering is more feasible to fabricate Cu 2 O photocathodes with less grain boundaries than electrodeposition (Figure7b).These works reported the improved PEC performance of a Cu 2 O photocathode by the mitigation of grain boundaries in the Cu 2 O photocathode, as shown in Figure7.

Figure 8 .
Figure 8. Energy level of the contact interface between Cu 2 O and metal.(a) Schottky contact (ϕ Cu 2 O > ϕ metal ) and (b) Ohmic contact (ϕ Cu 2 O < ϕ metal ) for hole migrations.Non-metallic materials have been explored as an alternative to the metallic back contact layer for Cu 2 O photocathodes.The main goal for these efforts is to develop the affordable Cu 2 O photocathode avoiding the usage of precious components.Nickel oxide (NiO) is a well-known material as a hole selective layer in perovskite solar cells[72][73][74].It is also possible to utilize the back contact layer of Cu 2 O photocathodes, because it has a larger work function (5.0 eV) than that of Cu 2 O.As shown in Figure 9a, it forms the Ohmic contact with Cu 2 O for smoothly migrating holes, while the fluorine-doped tin oxide (FTO) substrate forms the Schottky contact with Cu 2 O for interrupting the hole migration due to a shallow band barrier [75].Son et al. reported the copper nickel mixed oxide (CuO/NiO) hole selective layer for the Cu 2 O photocathode [76].They fabricated the CuO/NiO thin layer by a sequential metallic Cu/Ni sputtering and annealing process.It efficiently blocks the charge recombination at the interface between the Cu 2 O layer and the conductive substrates due to its huge energy barrier, resulting in an improved PEC performance (Figure 9b).Furthermore, it is quite transparent, which is beneficial for using the Cu 2 O photocathode as a top absorber in the tandem configuration.Copper thiocyanate (CuSCN) was applied as a back contact layer of a Cu 2 O photocathode in the work by Pan et al. [77].Although the hole transport from Cu 2 O into CuSCN is difficult due to the VB offset in terms of energy level, solution-processed CuSCN thin film facilitates the smooth hole transport from Cu 2 O into conductive substrates by the band-tail states existence, as illustrated in Figure 9c.Moreover, the huge barrier generated by the large CB band offset effectively prevents the charge recombination at the back contact interface.Zhou et al. suggested iron oxide hydroxide (FeOOH) as a hole transfer layer in the Cu 2 O photocathode [78].Electrodeposited FeOOH thin film promotes the extraction of holes

Figure 9 .
Figure 9. Energy band diagrams of Cu2O photocathodes with the non-metallic back contact layer.(a) Cu2O photocathodes-based FTO substrate and NiO back contact layer.Reprinted from [75] with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, copyright 2017.(b) Cu2O photocathodes with a CuO/NiO hole selective layer and Au back contact layer.Reprinted from [76] with permission from the Royal Society of Chemistry, copyright 2017.(c) Cu2O photocathodes with a solution-processed CuSCN back contact layer.Reprinted from [77] with permission from Pan et al., copyright 2020.(d) Cu2O photocathodes with an electrodeposited FeOOH hole transfer layer.Reprinted from [78] with permission from Elsevier B.V., copyright 2020.

Figure 9 .
Figure 9. Energy band diagrams of Cu 2 O photocathodes with the non-metallic back contact layer.(a) Cu 2 O photocathodes-based FTO substrate and NiO back contact layer.Reprinted from [75] with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, copyright 2017.(b) Cu 2 O photocathodes with a CuO/NiO hole selective layer and Au back contact layer.Reprinted from [76] with permission from the Royal Society of Chemistry, copyright 2017.(c) Cu 2 O photocathodes with a solution-processed CuSCN back contact layer.Reprinted from [77] with permission from Pan et al., copyright 2020.(d) Cu 2 O photocathodes with an electrodeposited FeOOH hole transfer layer.Reprinted from [78] with permission from Elsevier B.V., copyright 2020.
2 O photocathode.The optimization of the CuO/Cu 2 O heterostructured photocathode has continuously been explored by several groups.Du et al. derived the optimal annealing temperature (650 • C) for fabricating the highly efficient CuO/Cu 2 O photocathode [90].Jeong et al. found the optimal thickness of CuO for promoting the heterojunction effect [92].The CuO overlayer with a thickness of approximately 90 nm is optimal for improving the performance of the Cu 2 O photocathode by the CuO/Cu 2 O heterostructure, because it is close to the minority carrier diffusion length of CuO.Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 23 difference of EF and EF,redox further increases compared to the single Cu2O photocathode (Figure 10a,b)

Nanomaterials 2023 , 23 Figure 11 .
Figure 11.Stability change in the semiconductor in the water and redox potentials of Cu2O.It is drawn based on the information in the reported literature[95,96].

Figure 11 .
Figure 11.Stability change in the semiconductor in the water and redox potentials of Cu 2 O.It is drawn based on the information in the reported literature[95,96].

Figure 12 .
Figure 12.Efforts for improving the stability of Cu2O photocathodes using a TiO2 protection layer and its modification.(a) Long-term stability of Cu2O nanowire photocathodes with a Ga2O3 overlayer and RuOx HER catalysts.Reprinted from [87] with permission from Macmillan Publisher Ltd., Springer Nature, copyright 2018.(b) TiO2 surfaces before/after steam treatment and the durability of TiO2-protected Cu2O photocathodes assisted with low-temperature steam treatment.Reprinted from [97] with permission from the Royal Society of Chemistry, copyright 2014.(c) Crystallization of the TiO2 protection layer induced by laser irradiation and its effect on the stability of Cu2O photocathodes.Reprinted from [99] with permission from Elsevier B.V., copyright 2015.(d) Stability of Cu2O photocathodes with the modified TiO2 protection layer using a metallic nano filament design.Reprinted from [100] with permission from Wiley-VCH GmbH, copyright 2021.

Figure 12 .
Figure 12.Efforts for improving the stability of Cu 2 O photocathodes using a TiO 2 protection layer and its modification.(a) Long-term stability of Cu 2 O nanowire photocathodes with a Ga 2 O 3 overlayer and RuOx HER catalysts.Reprinted from [87] with permission from Macmillan Publisher Ltd., Springer Nature, copyright 2018.(b) TiO 2 surfaces before/after steam treatment and the durability of TiO 2 -protected Cu 2 O photocathodes assisted with low-temperature steam treatment.Reprinted from [97] with permission from the Royal Society of Chemistry, copyright 2014.(c) Crystallization of the TiO 2 protection layer induced by laser irradiation and its effect on the stability of Cu 2 O photocathodes.Reprinted from [99] with permission from Elsevier B.V., copyright 2015.(d) Stability of Cu 2 O photocathodes with the modified TiO 2 protection layer using a metallic nano filament design.Reprinted from [100] with permission from Wiley-VCH GmbH, copyright 2021.

Figure 13 .
Figure 13.Ni-based HER catalyst-decorated Cu 2 O photocathodes.(a) Schematic diagram of NiO x decorated ZnO/Cu 2 O photocathodes, HER catalytic activity of NiO x composite, PEC performance and stability of NiO x -decorated ZnO/Cu 2 O photocathodes.Reprinted with permission from [116].Copyright 2020 American Chemical Society.(b) Morphology and PEC performance of Ni-Modecorated Cu 2 O photocathodes with a TiO 2 protection layer.Reprinted from [117] with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2015.(c) PEC performance and stability of Ni-Fe LDH-decorated Cu 2 O photocathodes and bare Cu 2 O photocathodes.Reprinted from [118] with permission from Qi et al., copyright 2016.Nanomaterials 2023, 13, x FOR PEER REVIEW 17 of 23

Figure 14 .
Figure 14.Transition metal sulfides-and phosphides-catalyzed Cu2O photocathodes.(a) Schematic diagram and PEC performance of MoSx-decorated Cu2O photocathodes.Reprinted from [119] with permission from Springer Nature Limited, copyright 2014.(b) Schematic diagram and PEC performance of Al-NiS-catalyzed Cu2O photocathodes.Reprinted from [120] with permission from Elsevier B.V., copyright 2019.(c) Schematic diagram and PEC performance of CoP-decorated Cu2O photocathodes.Reprinted from [121] with permission from Elsevier Ltd., copyright 2017.(d) Morphology and PEC performance of a NiP-deposited CuO/Cu2O photocathode.Reprinted from [122] with permission from Royal Society of Chemistry, copyright 2018.Transition metal phosphides have also been explored as low-cost HER catalysts for Cu2O photocathodes because they have an excellent catalytic activity for HER.Cobalt phosphide (CoP) catalysts were successfully adopted for the Cu2O photocathode in the work by Stern et al.[121].Nanoflower-like CoP catalysts showed an extremely low HER overpotential of approximately 97 mV in a strong acidic condition to reach a current den-−2

Figure 14 .
Figure 14.Transition metal sulfides-and phosphides-catalyzed Cu 2 O photocathodes.(a) Schematic diagram and PEC performance of MoS x -decorated Cu 2 O photocathodes.Reprinted from [119] with permission from Springer Nature Limited, copyright 2014.(b) Schematic diagram and PEC performance of Al-NiS-catalyzed Cu 2 O photocathodes.Reprinted from [120] with permission from Elsevier B.V., copyright 2019.(c) Schematic diagram and PEC performance of CoP-decorated Cu 2 O photocathodes.Reprinted from [121] with permission from Elsevier Ltd., copyright 2017.(d) Morphology and PEC performance of a NiP-deposited CuO/Cu 2 O photocathode.Reprinted from [122] with permission from Royal Society of Chemistry, copyright 2018.

Table 1 .
Available metals as a back contact layer of the Cu 2 O photocathode and their work functions [66-69].

Table 1 .
Available metals as a back contact layer of the Cu2O photocathode and their work functions [66-69].
et al. reported that the gallium oxide (Ga 2 O 3 )/Cu 2 O heterostructure improves the photovoltage of Cu 2 O-based solar cells due to the decreased defect levels at the interfaces [85].Inspired by this work, Li et al. introduced the Ga 2 O 3 overlayer in the Cu 2 O photocathode for improving the PEC performance [86].The improved photovoltage by the Ga 2 O 3 /Cu 2 O heterostructure leads to a remarkable enhanced onset potential in the Cu 2 O photocathodes.Pan et al. further improved the PEC performance of Ga 2 O 3 overlayered Cu 2 O photocathodes with the Cu 2 O nanowire and ruthenium oxide (RuO x ) HER catalysts

Table 2 .
Onset potential and photocurrent density at the HER potential of heterostructured Cu 2 O photocathodes with n-type or p-type semiconductor overlayers.Some devices consist of a TiO 2 protection layer and HER catalysts, as well as overlayers.